A portable tissue equivalent calorimeter has been designed, constructed and applied to absolute dosimetric measurements in cyclotron produced fast neutron fields at both the Sloan-Kettering and Naval Research Laboratory cyclotrons. In addition, measurements were performed in a 60Co field at Massachusetts General Hospital and in the 160 MeV proton beam generated by the Harvard Cyclotron. This calorimetric dosimeter is presently under modification for field use at fast neutron radiotherapy trial centers. The calorimeter has been used for calibrations of other particle radiations. There is, at present, no national standard for absorbed dose in densely ionizing radiation fields. Calorimetry, which is a primary dosimetric method essentially independent of the neutron energy spectrum, has now been developed to a point where accurate field measurements can be carried out along with ionometric measurements performed using dosimeters normally employed at the high LET radiotherapy trial centers. These investigations will produce improved dosimetric accuracy and will also serve to measure dosimetric parameters which are at present, not well known.